Mixed aggregation between lithium diisopropylamide and lithium chloride: NMR, solid-state structure and ab initio calculations

An improved synthesis of (Pr¹ ₂NLi)₂LiCl(Me₂NCH₂CH ₂NMe₂)₂ (1) is presented. The structural characterisation of 1 by single crystal X-ray diffraction (C₂₄H₆₀ClLi₃N₆, M = 489.1, monoctinic P2₁/n, a = 9.126(3), b = 17.538(6), c = 20.470(7) Å, β = 98.02(4)°, V= 3244.2(19) ų, Z=4) revealed a three-runged Li-N, Cl-Li, Li-N ladder molecular structure. Cryoscopy in benzene indicated that 1 remained substantially undissociated in that solvent. Multinuclear variable temperature NMR in toluene revealed fluxionality; a mechanism of fluxion in arene solution involving rupture of an R₂N-Li bond followed by pseudorotation of all external ligands around a chloride fulcrum is proposed. Semi-empirical (MNDO) and ab initio (HF/6-31G *) calculational studies of the simplified model (LiNH₂)₂LiCl gave six-membered ring minimum energy geometries. While coordination of four water molecules to the model resulted in only modest changes to the six-membered ring structure at MNDO level, coordination of four dimethyl ether molecules led to a preference for the three-runged ladder structure as predicted by SCF calculations using a 6-31G * basis.